Z. B. Yan

Ho substitution suppresses collinear Dy spin order and enhances polarization in DyMnO3

The multiferroic behaviors of Dy1−xHoxMnO3 are investigated in order to reveal the effects of Ho-substitution on the independent collinear Dy spin order and ferroelectric polarization at low temperature. It is demonstrated that a partial Ho-substitution of Dy ions significantly suppresses the independent Dy spin order at low temperature on one hand, and maintains the R-Mn spin coupling on the other hand, thus leading to a remarkable enhancement of the polarization at low temperature.

Reversing ferroelectric polarization in multiferroic DyMn2O5 by nonmagnetic Al substitution of Mn

The multiferroic RMn2O5 family, where R is rare-earth ion or Y, exhibits rich physics of multiferroicity which has not yet well understood. DyMn2O5 is a representative member of this family. The ferroelectric polarization of DyMn2O5 is claimed to be magnetically relevant and have more than one component. Therefore, the polarization reversal upon the sequent magnetic transitions is expected. We investigate the evolution of the ferroelectric polarization upon a partial substitution of Mn3+ by nonmagnetic Al3+ in order to tailor the Mn3+-Mn4+ interactions and then to modulate the polarization in DyMn2−x/2Alx/2O5. It is revealed that the polarization can be successfully reversed by Al-substitution via substantially suppressing the Mn3+-Mn4+ interactions, while the Dy3+-Mn4+ interactions can sustain against the substitution until a level as high as x = 0.2. In addition, the independent Dy spin ordering is shifted remarkably down to an extremely low temperature due to the Al3+ substitution. The present work unv...

Multiferroicity and phase transitions in Tm-substituted GdMnO3

The multiferroic behaviors of polycrystalline Gd1−xTmxMnO3 are investigated by measuring the structural, magnetic, dielectric, and ferroelectric properties. Remarkable polarization is observed by partial Tm-substitution of Gd ions, and can be proposed to arise from the contribution of both the symmetric exchange striction (S·S term) and asymmetric exchange striction (S × S term) in the ab-plane cycloidal structure at low substitution level. With further substitution, the polarization is gradually reduced, and it is suggested that the ferroelectric phase evolves from the ab-plane cycloidal structure to the bc-plane cycloidal structure where only the antisymmetric exchange striction (S × S term) contributes to the polarization.

Enhanced ferromagnetism, metal-insulator transition, and large magnetoresistance in La1−xCaxMn1−xRuxO3 free of eg-orbital double-exchange

The structure, ionic valences, magnetism, and magneto-transport behaviors of mixed valence oxides La1−xCaxMn1−xRuxO3 are systematically investigated. The simultaneous substitutions of La3+ and Mn3+ ions by Ca2+ and Ru4+, respectively, are confirmed by the structural and ionic valence characterizations, excluding the presence of Mn4+ and Ru3+ ions. The enhanced ferromagnetism, induced metal-insulator transition, and remarkable magnetoresistance effect are demonstrated when the substitution level x is lower than ∼0.6, in spite of the absence of the Mn3+-Ru4+ eg-orbital double-exchange. These anomalous magnetotransport effects are discussed based on the competing multifold interactions associated with the Mn3+-Ru4+ super-exchange and strong Ru4+-Ru4+ hopping, while the origins for the metal-insulator transition and magnetoresistance effect remain to be clarified.

Magnetic properties and electronic structures of (YTiO3)2/(BaTiO3)n superlattices

The magnetic properties and electronic structures of (YTiO3)2/(BaTiO3)n superlattices are investigated using the first-principles calculations. It is revealed that the in-plane compressive strain results in the A-type antiferromagnetic order in the YTiO3 component. Surprisingly, the Ti ions in BaTiO3 layers exhibit a weak ferromagnetic order for n = 4. The ferromagnetism in the BaTiO3 layers near the interface is related to the polar discontinuity of YTiO3 and ferroelectric polarization of BaTiO3. The electronic structures indicate that the n = 4 superlattice shows the two-dimensional electron gas at the interface.

Large and Fast Single-Crystal Resistive Humidity Sensitivity of Metal Pnictide Halides Containing van der Waals Host–Guest Interactions

Two new metal pnictide halides, (Hg(9.75)As(5.5))(GaCl4)3 and (Hg13Sb8)(ZnBr4)4, have been prepared by solid-state reactions. Their structures feature 3D cationic host frameworks built of mercury pnictide polyhedra and form 1D tunnels filled with discrete guest halide polyanions; the guests and hosts are assembled by van der Waals interactions. Both complexes exhibit good single-crystal humidity sensitivity, with a humidity sensitivity factor as big as three orders of magnitude, a quick resistance response, fast recovery, and good reproducibility. This study provide a new way to design promising resistive humidity detectors by introducing van der Waals host-guest interactions into their structures.

Antiferroelectric polarization switching and dynamic scaling of energy storage: A Monte Carlo simulation

The polarization-electric field hysteresis loops and the dynamics of polarization switching in a two-dimensional antiferroelectric (AFE) lattice submitted to a time-oscillating electric field E(t) of frequency f and amplitude E0, is investigated using Monte Carlo simulation based on the Landau–Devonshire phenomenological theory on antiferroelectrics. It is revealed that the AFE double-loop hysteresis area A, i.e., the energy loss in one cycle of polarization switching, exhibits the single-peak frequency dispersion A(f), suggesting the unique characteristic time for polarization switching, which is independent of E0 as long as E0 is larger than the quasi-static coercive field for the antiferroelectric–ferroelectric transitions. However, the dependence of recoverable stored energy W on amplitude E0 seems to be complicated depending on temperature T and frequency f. A dynamic scaling behavior of the energy loss dispersion A(f) over a wide range of E0 is obtained, confirming the unique characteristic time for...

Spin frustration destruction and ferroelectricity modulation in Ca3CoMnO6: Effects of Mn deficiency

We present careful experiments on the spin order and multiferroicity of Mn-deficient Ca3CoMn1−xO6 with respect to the Co/Mn compensated Ca3Co1+yMn1−yO6. It is revealed that a slight destruction of the Co/Mn ionic order does not damage but significantly enhances the ferroelectricity in terms of the transition point and polarization magnitude, due to the Mn-deficiency enhanced degree of Co/Mn spin ordering in comparison with Ca3Co1+yMn1−yO6. The delicate competition between the ionic (charge) order and spin frustration is the core physics for improving the ferroelectric performance.

Magnetic phase transitions and monopole excitations in spin ice under uniaxial pressure: A Monte Carlo simulation

In this work, we explore the spin ice model under uniaxial pressure using the Monte Carlo simulation method. For the known spin ices, the interaction correction (δ) introduced by the uniaxial pressure varies in quite a wide range from positive to negative. When δ is positive, the ground state characterized by the ferromagnetic spin chains is quite unstable, and in real materials it serves as intermediate state connecting the ice state and the long range ordered dipolar spin ice ground state. In the case of negative δ, the system relaxes from highly degenerate ice state to ordered ferromagnetic state via a first order phase transition. Furthermore, the domain walls in such ferromagnetic state are the hotbed of the excitations of magnetic monopoles, thus indicating that the uniaxial pressure can greatly increase the monopole density.

Electric field control of ferroelectric domain structures in MnWO4.

Competing interactions make the magnetic structure of MnWO4 highly frustrated, and only the AF2 phase of the three magnetically ordered phases (AF1, AF2, AF3) is ferroelectric. The high frustration may thus allow a possibility to tune the magnetic structure by means of an electric field via magnetoelectric coupling. By using the pyroelectric current method, we measure the remnant ferroelectric polarization in MnWO4 upon application of a poling electric field via two different roadmaps. It is demonstrated that an electric field as low as 10 kV cm(-1) is sufficient to enhance the stability of a ferroelectric AF2 phase at the expense of a non-ferroelectric AF1 phase. This work suggests that electric field induced electrostatic energy, although small due to weak magnetically induced electric polarization, may effectively tune ferroelectric domain structures, and thus the magnetic structure of highly frustrated multiferroic materials.